Former Mayor Richard Riordan is calling for a criminal investigation after an incident involving his pension reform plan for Los Angeles city workers.

An organizer with the Service Employees International Union Local 721, an engineers' union, emailed a letter to members instructing them to sign fake signatures and addresses on petitions for Riordan's plan.

The fake signatures could, in theory, be used to disqualify Riordan's effort to get a reform measure on the May ballot.

The measure would move newly hired city workers off taxpayer-guaranteed pensions and onto 401(k)-style accounts. There needs to be 265,151 valid signatures for the measure to get on the ballot.

The union has come out against the email, saying the organizer has been disciplined.